The present invention relates to dynamoelectric machines and, more particularly, to apparatus for sensing one or more shorted turns in a rotor field of a large dynamoelectric machine.
A large synchronous dynamoelectric machine such as, for example, a large turbine-generator, employs the interaction of magnetic fields in its rotor and stator to produce electric power. In order to simplify the high-power output connections, the high AC output power is conventionally generated in the stator which operates as an armature. A plurality of field windings on the rotor are conventionally excited by a DC field supply. The DC field supply is generated either by an external DC exciter generator and fed through slip rings to the rotor field windings, or in a brushless generator-rectifier assembly rotating with the rotor.
Each of the field windings consists of a plurality of mutually insulated conductor bars disposed in slots in the rotor where they are retained by radial force generated by dovetail wedges fitted into dovetail slots in the rotor. End turns interconnecting the ends of the conductor bars are held in place by retaining rings which may be thermally shrunk or fitted into place. The dovetail wedges and the retaining rings support the field windings and end turns, respectively, against centrifugal forces tending to urge them outwardly.
Rotors of large turbine-generators conventionally have two, four or more poles formed by the arrangement of the slots containing the field windings and by the method of interconnection. For purposes of concreteness of description, a two-pole turbine-generator is illustrated and described herein, it being understood that the invention is not limited to such an embodiment.
A two-pole rotor includes a plurality of slots disposed parallel to the axis of a cylindrical forging. A conventional rotor may contain from as few as 8 to as many as 18 or more slots on each side of a pole axis bisecting the rotor. The slots are symmetrically arranged with respect to the pole axis and the slots and windings are arranged to produce a flux field which approximates a sine wave in space. Each slot contains a plurality of mutually insulated bars. In one generator, for example, a total of 14 slots are employed on each side of the pole axis whose conductor bars are interconnected by end turns to create seven coils per pole. The four slots nearest the poles each coontain seven conductor bars and the remainder of the slots each contain eleven conductor bars.
The insulation separating the conductor bars or end turns of a rotor of the above type may break down for reasons which are not of concern to the present discussion. Such an insulation breakdown may place a short circuit across one or more of the turns in a winding. Short circuited turns may exist at a standstill or may only occur when the conductor bars or end turns are outwardly loaded by centrifugal forces. The former may be detected by static tests whereas the latter may only be detected in a rotor turning at or near its operational speed.
U.S. Pat. No. 3,506,914, the disclosure of which is herein incorporated by reference, describes a method and apparatus for sensing the rate of change of radial and tangential flux as each slot in a field rotor passes a search coil. In summary, a search coil arranged to sense radial and/or tangential flux is disposed close to the surface of the spinning rotor. Leakage flux from the rotor slots induces voltages in the search coil as the slots rotate therepast. The induced voltages from the search coil drive an oscilloscope. Irregularities in the pattern of peaks and valleys displayed on the oscilloscope is interpreted in a disclosed manner to determine whether one or more turns are shorted and to identify the slot or slots in which the shorted turns are located. The technique is considered sensitive enough to determine the number of shorted turns associated with a rotor slot.
The high economic value of a large turbine-generator, and the high cost of replacing its power during an outage, strongly encourage maintaining the turbine-generator in operation as long as safe operation can be continued. The same reasons make it valuable to achieve repair and service resumption with minimum delay. A small number of shorted field turns is not generally considered to represent a catastrophic failure requiring taking the generator off line and attempting a repair. Instead, satisfactory operation may be continued with one or more shorted turns.
Besides reducing the field flux generated by the conductor bars in one or more slots, a shorted turn also changes the power dissipated in the affected winding. Such changes in power dissipation can produce non-uniform heating of the rotor and result in thermally-induced distortion and rotor vibration. When the pattern of shorted turns in the rotor slots permits it, such rotor vibration is preferably cured by balancing. If the pattern of shorted turns is asymmetrically distributed on the rotor, such balancing may be ineffective and a disassembly and repair of the affected conductor bars may be required. Such disassembly and repair is not undertaken lightly since it consumes a significant amount of time and invokes a probability that the conduct of the work may create faults which were not present before the start of the work.
The flux changes sensed by the apparatus of the above-referenced patent provides information which can guide the decision between balancing and repair. In addition, the information is sufficient to identify individual slots containing shorted turns, as well as the number of shorted turns contained therein.
The referenced patent requires the disposition of the search coil within a relatively short distance from the peripheral surface of the rotor. Best operation requires a radial spacing of the search coil from the peripheral surface of the rotor of from about 0.5 to about 1.00 inch. Since the retaining rings at the ends of the rotor have a radius of about 1.5 inch greater than the radius of the rotor, a search coil installed at the required spacing during manufacture of the stator of the turbine-generator would be destroyed by the retaining rings when the rotor is installed. The referenced patent provides a temporary probe which is inserted through a gas-tight sealing gland in the wrapper of the turbine-generator and through one of the core tooth vent ducts in the stator to place its extremity containing the search coil at the required distance from the rotor peripheral surface. Since the probe interferes with cooling gas flow through the core tooth vent duct which is occupies, it is desirable to limit the use of the measurement technique to short periods. Extended operating periods could also produce vibration damage to the probe.
Although, as noted above, a small number of shorted turns in a field rotor is not generally considered to be a serious problem, continued refinement of electric power-generation and control equipment, and the availability of techniques for analyzing and recording the data therefrom, makes it desirable to provide a flux search probe which can permanently be installed in the stator during the manufacture thereof and which can withstand the subsequent assembly and operational environments, particularly the installation of the rotor, without damage to the probe or to the turbine-generator.